Physiological responses to waterlogging in barley (Hordeum vulgare L.)

Waterlogging is a major environmental constraint severely limiting crop production both in Australia and worldwide. In Australia, the problem is especially acute in higher rainfall areas with poorly draining soils. With most Australian commercial barley cultivars being waterlogging sensitive, waterlogging tolerant genetic material has to be selected and used in the breeding programs. Of specific interest is Chinese germplasm collection. However, lack of appropriate screening methodology makes selection ofwaterlogging tolerant cultivars difficult. Also, waterlogging is a complex abiotic stress encoding a large number of physiological and biochemical mechanisms and complicated by many confounding factors. Accordingly, the aims of this project were three-fold: (1) to develop an efficient screening methodology for waterlogging tolerance in barley breeding programs; (2) to select some waterlogging tolerant cultivars among existing barley germplasm; and (3) to understand the physiological, morphological and anatomical mechanisms encoding waterlogging tolerance in barley. The project was an integral part ofGRDC-funded collaboration between Australia and China on barley genetics. Barley plant growth was adversely affected by waterlogging. As waterlogging stress developed, chlorophyll content, net C02 assimilation and maximal photochemical efficiency of PSil (Fv/Fm) decreased significantly. Among these, chlorophyll fluorescence of dark-adapted samples (Fv/Fm values) was found to be the most efficient screening parameter for large-scale programs for waterlogging tolerance. Among studied genotypes, the Chinese cultivar TX9425 was found to be the most waterlogging tolerant, with the least reduction of plant growth, chlorophyll content, chlorophyll fluorescence and photosynthetic parameters. Naso Nijo (a Japanese cultivar) and Franklin (an Australian cultivar) were found to be the most susceptible to waterlogging. The adverse effects in stressed plants were alleviated following 2 weeks recovery in all cultivars. These contrasting genotypes were used later in all physiological studies. Overall, this study suggests that substantial genetic potential exists to improve waterlogging tolerance in barley. Measuring chlorophyll fluorescence of darkadapted samples (Fv/Fm values) is recommended as an efficient screening tool for waterlogging tolerance in barley breeding program. Key features targeted by breeding should include both morphological (production of more adventitious roots and formation of larger aerenchyma area in adventitious roots) and physiological (high ability of 0 2 uptake and K+ acquisition in plant roots) traits, as well as the plant's ability to withstand soil-borne phytotoxins. Foliar application of nutrient c~ be used in practice to alleviate the adverse effects ofwaterlogging